In LTE network when MME triggers for tracking area update to UE?

Question

There are two ways for tracking area update procedure to get triggered from UE to network . One is known as periodic and other one as non-periodic.

Answer 1

Answering the question as it's stated then.. it's basically Never. I haven't heard about case of relocating GUTI, although it sounds logically OK.

When the UE triggers the TAU?

1. UE detects it has entered a new TA (Tracking Area) that is not in the list of TAIs (Tracking Area Indicators) that the UE registered with the network;
2. The periodic TA update timer has expired;
3. UE was in UTRAN PMM_Connected state (e.g. URA_PCH) when it reselects to E-UTRAN;
4. UE was in GPRS READY state when it reselects to E-UTRAN;
5. The TIN indicates "P-TMSI" when the UE reselects to E-UTRAN (e.g. due to bearer configuration modifications performed on GERAN/UTRAN);
6. The RRC connection was released with release cause "load re-balancing TAU required";
7. The RRC layer in the UE informs the UE's NAS layer that an RRC connection failure (in either E-UTRAN or UTRAN) has occurred;
8. When the UE changes the radio capability of at least one of the following radio access technologies: GERAN, UTRAN or CDMA 2000;
9. For a UE supporting CS fallback, or configured to support IMS voice, or both, a change of the UE's usage setting or voice domain preference for E-UTRAN.
10. For a SR-VCC capable UE, a change of MS Classmark 2 and/or MS Classmark 3 and/or Supported Codecs.
11. UE manually selects a CSG cell whose CSG ID is absent from both the UE's Allowed CSG list and the UE's Operator CSG list.

For more on TAU please go to my blog
http://www.lteandbeyond.com/2012/04/tracking-area-update-tau-procedure.html

Answer 2

MME can force UE to initiate TAU procedure via MME relocation procedure in order to reallocate the GUTI or redistribute the MME Loads.

The other case would be the case of load balancing where MME will use the re-balancing function that forces the UEs to reattach to another MME by using a cause “Load Balancing TAU Required” in the S1AP-UE Release Command to enodeB. After receiving it EnodeB sends RRC Connection Release with cause "Load balancing TAU required" to UE. Upon receiving, UE can/will initiate TAU procedure.

Comments

Can you please provide any spec reference ?
Thanks.

Answer 3

I think that UE only detect new tracking area and sends tracking area update to MME. I never heard or read that MME triggers TAU procedure for an UE.

There are two types of trigger for TAU.
1. Periodic TAU:
In case of periodic TAU, UE sends TAU request to MME irrespective of any change in current tracking area.
2. A-periodic TAU:
whenever an UE detects a new tracking, it reports to MME when UE is in RRC idle mode.

Answer 4

Spec 24.301 will have answers to your question.